Modern wireless data transmission systems such as WiMax, WiMax-II, and 3GPP LTE are designed to reduce wireless channel distortions, such as fading, that may cause errors to appear in data even at low noise levels. Common methods of reducing errors include forward error correction (FEC) and hybrid automatic repeat request (HARQ). HARQ is usually implemented in one of two schemes.
The first HARQ scheme implements a simple data retransmission. During the retransmission, the same bits are re-sent and the receiver side combines two or more received packets during demodulation. A common implementation of the simple data retransmission is Chase-combining, or CC-mode.
The other HARQ scheme implements an Incremental Redundancy (IR) scheme, or IR-mode. This scheme utilizes specific features of the error correcting code. It allows decoding having only part of the data word that is transmitted during the first attempt. If decoding fails, the transmit side transmits additional parts of the data word, and the decoder on the receiver side attempts to decode using a combination of the first and subsequent retransmissions to recover from errors. In general, an incremental redundancy method shows improved performance over simple Chase-combining, with the added cost of some additional system complexity.
Customary to bit-to-symbol mappings implemented in HARQ-allowed systems, the same bit-to-symbol mapping is used for each retransmission attempt. Symbol Mapping Diversity (SMD) proposes a slight modification to the bit-to-symbol mapping applied to an entire packet for each subsequent retransmission. If there is any uncertainty in the results at the receiver side, then, in the retransmission, a new mapping is applied to the entire retransmission packet in an attempt to reduce the number of retransmissions. In WiMAX, the IR packet during any retransmission may hold both bits that are going to be transmitted for the first time, as well as some bits that have already been transmitted in a previously transmitted packet. Thus, selecting an entire retransmission packet and applying an SMD mapping to the entire retransmission packet based on the number of retransmissions leads to a correct mapping for only some of the bits in the IR packet, showing no performance increase.
Throughout the description, similar reference numbers may be used to identify similar elements.